Abstract
Using the JMA global spectral model a forecast experiment was performed to investigate the mechanisms to maintain the Baiu (Meiyu) front and the air masses around it. The initial condition was June 15, 1984 and the model was integrated 15 days until June 30.
Over the China continent the Meiyu front, which is characterized by a stationary band of large humidity contrast, was well predicted by the model. When the Tibetan Plateau was Tibet and propagated removed from the model, a baroclinic wave developed in the eastward. It disturbed the stationary Meiyu front. This suggests one of the roles of the Tibetan Plateau is to prevent from mixing the warm, moist air to the south and the cold, dry air to the north of it and to suppress development of baroclinic disturbances. This situation is a favorable condition for the Meiyu front to be stationary.
In the model the Baiu front shifted northward around Japan and the north Pacific Ocean and the subtropical high extended westward and northward compared with the observation. The polar air mass north of the front became warm and the Aleutian low weakened substantially.This error was found in whole troposphere below 300 mb. The cooling effect due to the low sea surface temperature over the north Pacific Ocean was investigated by integrating a model with artificial this strong cooling in the lower atmosphere. However,only improved the temperature in the lower troposphere. The temperature field and circulation did not change in the middle and upper troposphere. Heat budget analysis in the north Pacific Ocean using observed data showed that the cold air advection associated with the lows which develop around the Aleutian Islands is equally or more important than the cooling from the sea surface to maintain the cold polar air mass. Especially in the middle and upper troposphere the southward cold air advection is the major contributing factor.
There was a positive temperature error in the Siberia and the north China in the model, which was due to the excessive sensible heat flux. This was related to a small value of the efficiency of the surface evaporability used in the model. Increase of the parameter can reduce the temperature error over the land. However, the air in the PBL became wetter than the observation. It suggests that an appropriate treatment of the process at the ground surface is important to predict correctly the characteristics of the air mass north of the Meiyu front.
